The invention relates to layers comprising polyimide and a functional material such as a hole transport material (hole conductor material), an electron transport material (electron conductor material) or an emitter material. Polyimide layers doped with said functional materials can be used for the production of novel electronic and optoelectronic devices.
The use of polyimide layers as polyimide alignment layers to orient liquid crystalline materials is a standard technology in the liquid crystal (LC) display industry. A review of the state of the art in orienting layers for liquid crystals can be found in Ref. [1].
Polyimides for alignment layers are typically prepared from polyamic acid or polyamic ester precursor compounds. One example of a precursor polyimide is ZLI 2650 from Merck. Usually, the precursor polymer is first deposited and subsequently chemically or thermally converted into an insoluble polyimide. Alternatively, soluble preconverted polyimides can be used.
The orientation layer is prepared in order to give it aligning properties. Conventionally, surface preparation is made by rubbing with a cloth. Other methods for surface preparation known in the art make use of polarized ultraviolet light or flows of hot air. The liquid crystals are subsequently aligned by depositing them onto the alignment layer and letting them align at a temperature above the melting point and below the clearing point of the materials.
However, polyimide films are very poor hole or electron conductors. Thus, if layers are required which have a high hole or electron transport and injection ability, as in the case discussed in the following paragraph, undoped polyimide films would reduce the efficiency of the respective devices.
On the other hand, light-emitting devices (LEDs) based on organic and polymeric electroluminescent materials are known. Typically such devices require two electrodes of differing work function, at least one of which is transparent. A high work function anode (e.g. indium tin oxide (ITO), fluorine-doped tin oxide (FTO), or gold) serves for hole injection and a low work function cathode (e.g. Mg, Al, Li, Ba, Yb, Ca) for electron injection into the organic or polymeric material.
In order to make a high efficiency LED, it is desirable for current in the device to be space charge limited instead of injection limited. As will be described in detail below, a space charge limited current is indicated in J*d versus V/d plots by the independence of the plot from the interelectrode distance d.
For realizing space charge limited current, the barriers for charge injection have to be small. One way to achieve this is to fabricate devices comprising additional layers to facilitate injection and transport of electrons and holes. One type of layer comprises organic hole conductors, typically aromatic amines, which facilitate the injection and transport of holes in the device.
These materials are typically used in the undoped state and represent organic semiconductors with a low intrinsic conductivity. Such semiconducting hole conductor layers can increase the efficiency of the device by balancing injection and charge of positive and negative charge carriers and forcing the recombination of charge carriers to occur in a region well away from the electrodes, thereby preventing the emission efficiency from being reduced by semiconductor/metal interactions. The use of hole conducting layers in polymeric and organic LEDs is described e.g. in Refs. [2] and [3].
In most cases the multilayers are prepared by organic evaporation to avoid dissolving the previously applied layer, which can happen when solvent-based processing such as spin coating, doctor blading, dip coating, etc. are used.
Alternatively, metallically conductive doped conjugated polymers such as polythiophene or polyaniline may be used as anode modification layers to adjust the work function of the electrode and to provide a stable interface between semiconductor layer and anode. The use of such electrodes is described in Refs. [4] and [5].
The use of dendrimeric structures (Ref. [6]), sterically hindered structures using spirobifluorene or triptycene units (Refs. [3] and [7]) and crosslinkable structures (Ref. [8]) having a relatively high glass transition temperature (glass temperature, Tg) and forming a stable glassy phase, in organic LEDs has been described. The glass transition temperature is phenomenologically characterized by the transition from a more or less hard, amorphous glass-like or partially crystalline state into a rubber-like to viscous melt-like state.
In recent years, the possibility of realizing polymeric LEDs emitting polarized light has been investigated. Thereby, polarized emission has been achieved to some extent by stretch orienting emitting, non liquid crystalline polymers (Ref. [9]). Also in this reference, the possible use of polarized polymer LEDs as backlights for liquid crystalline displays is addressed. In Ref. [10], polarized emission from rigid-rod molecules which were aligned by the Langmuir-Blodgett transfer method is described. In both cases, no orientation in a liquid crystalline phase was used to achieve polarized alignment.
A polymer LED in which a layer of polythiophene or polyaniline was rubbed and used to align a low molecular weight liquid crystal doped with a small amount of dichroic fluorescent dye is described in Ref. [11]. However, the aligning ability of polythiophenes is known to be not high and, indeed, alignment of polymeric liquid crystals on rubbed polythiophene was not shown.
Polarized emission was obtained on rubbed poly(p-phenylenevinylene) according to Ref. [12]. LEDs comprising polymeric liquid crystalline emitter materials aligned on rubbed polyimide are disclosed in Ref. [13]. However, polyimide cannot serve as a hole or electron conducting material. In Ref. [14], polarized electroluminescence by epitaxial growth of sexiphenylene onto a rubbed sexiphenylene film is described.
The use of a polyimide derivative in a photoconductive device is described in Ref. [15]. The polyimide derivative is a polyimide comprising, covalently linked, a moiety in the main chain that might serve as a hole conducting compound.
Finally, a hole conducting layer prepared by mixing the hole conductor N,Nxe2x80x2-diphenyl-N,Nxe2x80x2-di(m-tolyl)benzidine (TPD) with a polyamic acid precursor in the ratio 50/50 by weight, followed by thermal conversion, is described in Ref. [16]. TPD has a glass transition temperature (Tg) of 65xc2x0 C. and a molecular weight of below 500.
As described in Ref. [16], a two-layer LED can be made with the above TPD/polyimide layer by vacuum deposition of an aluminum complex as electron transport and emitting material. However, at concentrations high enough to produce good hole transport, the hole conductor TPD and the polyimide phase separate and the TPD crystallizes. This brings about the problem, firstly, that the phase separated TPD is susceptible to attack by solvents in subsequent coating processes and, secondly, that the film cannot be processed to be an orientation layer, especially by rubbing, as rubbing of the layer comprising crystallized TPD would result in severe damage.
As mentioned above in the context of characteristics of polyimide films, polyimide itself has only poor hole or electron transport and injection properties. Thus, the use of a polyimide layer in an undoped state reduces the efficiency of devices in which hole/electron transport is needed.
On the other hand, although some alignment can be achieved by rubbing electrically conducting or hole transporting polymers, these materials do not have such a high degree of aligning ability as polyimide. Thus, alignment layers based on such polymers are unsatisfactory.
In addition, one is restricted to the energy levels and hole mobilities provided intrinsically by the materials, although in principle one wishes to be able to adjust the energy levels and mobilities independently of each other, as well as the aligning ability of the material.
Thus, by dividing the energy barrier for hole injection into several small barriers by stepwise deposition of materials with, different energy levels, an improvement of hole injection efficiency should be possible. However, currently this cannot be done with solvent processed films because of cosolubility. In other words, application of an additional layer onto a previously applied layer by a method employing a solution of the material to be applied in a solvent would result in a dissolution of said previously applied layer.
Therefore, in the production of multilayer electroluminescent devices showing high efficiency and whose color can be tuned by choosing different chromophores, one is limited to low molecular weight materials which are deposited by organic physical vapour deposition under high vacuum. Consequently, only a few of such systems, which are not easily changed, are available. However, it would be desirable to replace said low molecular weight materials with robust, flexible polymers as well as to replace vacuum deposition with low-cost solvent based coating technologies. As described above, this is not possible with commenly used solvent based methods according to the state of the art because the first layer is dissolved by the solvent used for depositing subsequent layers.
Thus, in view of the above, it is an object of the invention to provide a novel layer for use in electronic and optoelectronic devices that provides good aligning ability as well as good hole or electron transport or emitter ability, whose aligning and conductor ability can be easily and independently tuned, and which can be processed for alignment with standard techniques, such as rubbing. Furthermore, the layer should be resistant to standard organic solvents, such as toluene, THF, methylene dichloride etc., thus allowing stepwise deposition of additional layers thereon by methods other than vacuum deposition techniques.
Furthermore, it is an object of the invention to provide electronic and optoelectronic devices making use of the above layers, and a method of producing them.
According to the invention, there is provided a layer comprising polyimide and one or more organic materials selected from the group consisting of hole transport materials, electron transport materials and emitter materials, wherein the hole transport, electron tranport or emitter material has a Tgof higher than 80xc2x0 C.
Said layer is obtainable by mixing a hole transport, electron transport or emitter material or a combination thereof with a polyimide precursor material, forming a thin film out of the mixture and converting said mixture into a thin film of doped polyimide.
Preferably, the hole transport, electron transport or emitter material used in the layer according to the invention has a molecular weight of greater than 750 and, more preferably, a Tg of higher than 100xc2x0 C. and/or a molecular weight of greater than 900.
Furthermore, the content of hole transport, electron transport or emitter material (i.e. the doping material) in the mixture of doping material and polyimide precursor material used to prepare said layer is preferably selected in the range of 1% to 60% by weight, more preferably in the range of 5% to 50% by weight, and most preferably in the range of 15% to 35% by weight, based on the total amount of doping material and polyimide precursor material.
The layers according to the invention are, after conversion to polyimide, thermally stable up to well over 100xc2x0 C. without morphological or chemical changes. They are homogenous, do not phase separate and can be easily processed to have aligning properties by standard techniques such as rubbing, UV irradiation, treatment with an air flow, etc.
The layers combine the qualities of good alignment materials on the one hand and of good hole transport, electron transport or emitter materials, respectively, on the other hand.
By varying the concentration of e.g. the hole transport material in the mixture with polyimide, the mobility can be tuned to the optimal value for the device in question. By doping with different hole conductors the energy levels can be tuned for optimal hole injection.
It has been found that by using the films according to the invention, it is possible to obtain space charge limited current over a wide range of concentrations and mobilities.
In addition, the homogeneously mixed composite layers according to the invention are resistant after conversion to polyimide to further solvent treatment, thereby allowing the preparation of multilayers by successive coating and conversion cycles. This allows on the one hand the preparation of successive hole injection and conducting layers with small energy barriers to improve hole injection. It also allows on the other hand the preparation of e.g. multilayer electroluminescent devices with tunable mobilities and energy levels by solvent based coating processes.
Thus, the layers according to the invention combine all of the above advantages of stability, charge injection and transport, tunability and solvent resistance with a high aligning ability.
Furthermore, there are provided according to the invention electronic and optoelectronic devices according to claims 10, 12 and 16 and a method of manufacturing said devices according to claim 17, the method and devices making use of the above layers of the invention.